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The Reproductive Behaviour and the Nature of Sexual Selection in Scatophaga stercoraria L. (Diptera: Scatophagidae): II. The Fertilization Rate and the Spatial and Temporal Relationships of Each Sex Around the Site of Mating and Oviposition

G. A. Parker
Journal of Animal Ecology
Vol. 39, No. 1 (Feb., 1970), pp. 205-228
DOI: 10.2307/2896
Stable URL: http://www.jstor.org/stable/2896
Page Count: 24
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
The Reproductive Behaviour and the Nature of Sexual Selection in Scatophaga stercoraria L. (Diptera: Scatophagidae): II. The Fertilization Rate and the Spatial and Temporal Relationships of Each Sex Around the Site of Mating and Oviposition
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Abstract

(1) `Take-overs' (where a second male dung-fly (Scatophaga stercoraria) takes possession of a paired female by ousting the original male) cause an average extension of about 12 min in peak seasons and 3 min in declines to the average time spent by the females at the dung, which is normally about 45 min (at 20 degrees C) for mating and oviposition. (2) A character of selective advantage in terms of sexual selection (fighting for females) may therefore evolve even though it appears to lower the overall reproductive rate of the species (by increasing the generation time). (3) The fertilization rate (number of eggs fertilized by a male in unit time) achieved by males around the dropping is about 0.23 eggs/male/min, hence the selective disadvantage (in terms of sexual selection) to a male of `time waste' at the dropping would be considerable. (4) The average `search time' (time taken for a male to obtain a female at the dropping) is about 140 min. (5) The average fertilization rate of male S. stercoraria is dependent on the sex ratio around the dung, which is apparently unaffected by seasonal changes in population density. (6) It is postulated that in conditions of sexual competition where males are searching for females, an evolutionary `equilibrium position' should eventually be established where all males achieve an approximately equal fertilization rate. The fertilization rate per male in S. stercoraria appears to be in equilibrium in that it is equally distributed with time after the dropping is deposited, i.e. temporal equilibrium is established. (7) Males around cattle pastures appear to mate about three times per day, and the duration of copula of a given male decreases with successive matings but returns to normal the next day. (8) Searching for females occurs both on the dropping and in the surrounding grass. About 90% of the males searching in the grass do so up-wind of the dropping. Females appear usually to fly over the dropping and then walk towards it, with the wind. (9) The `pat change interval' (time taken for a male to find a fresh pat) probably approximates to 4 min. (10) The observed relationship between time spent searching on the dung surface and that spent in the surrounding grass may also represent an `equilibrium distribution' of fertilization rate whereby equal fertilization rates are achieved in the two localities. Thus spatial equilibrium may also be established, though this remains to be analysed quantitatively.

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